1. Field of the Invention
The present invention relates to microfine ethylene copolymer powders which are crosslinkable and wherein the powder particles are spherical or substantially spherical in shape. The invention also relates to a process for producing and crosslinking the polymer powders.
2. Description of the Prior Art
The use of thermoplastic resin powders is well documented in the prior art. For example, powdered thermoplastic resins in dry form have been used to coat articles by dip coating in either a static or fluidized bed and by powder coating. Powders can also be applied in dispersed form, by roller coating, spray coating, slush coating, and dip coating substrates such as metal, paper, paperboard, and the like. Powders are also widely employed for conventional powder lining and powder molding processes, e.g., rotational molding. Still other applications for powders include use as paper pulp additives; mold release agents; additives to waxes, paints, caulks, and polishes; binders for non-woven fabrics; etc.
Besides the physical properties of the powder, which are dictated by the resin being used, the size and shape of the particles are the other major properties which influence the selection of a powder for various applications. These latter properties are primarily a function of the process by which the powders are prepared, which can include mechanical grinding, solution processes and dispersion processes. Particle size is determined using U.S. Standard Sieves or light scattering techniques and, depending on the method used, will be reported in mesh size or microns. The inverse relationship between the sieve size (mesh number) and particle size (in microns) is well documented and conversion tables are available The shape of the particles is ascertained from photomicrographs of the powders. Particle shape has a marked influence on the bulk density of the powder and its handling properties.
For most effective fluidization and dry spraying, it is generally considered advantageous to use powders which have a fairly narrow particle size distribution and wherein the particles are spherical in shape. Powders produced by mechanical grinding or pulverization, which are typically irregular in shape and generally have quite broad particle size distributions, are not well suited for fluidization and dry spraying. While the particles of powders produced by solution processes are less irregular than those obtained by mechanical means, they are still not spherical.
Powders obtained using dispersion techniques, such as those described in U.S. Pat. Nos. 3,422,049 and 3,746,681, wherein the particles produced are spherical in shape and fall within a relatively narrow size range are most advantageously employed for fluidization and dry spraying. These processes involve dispersing a molten synthetic organic polymeric thermoplastic resin in about 0.8 to 9 arts by weight of water per part of resin in the presence of from about 2 to 25 parts by weight per 100 parts of resin of a water-soluble block copolymer of ethylene oxide and propylene oxide having a molecular weight above about 3500 and containing at least about 50% by weight of ethylene oxide and in the absence of an organic solvent for the polymer. The fine dispersion which is produced is then cooled to below the softening temperature of the resin to obtain the powder.
A continuous process for the preparation of finely divided polymer particles is disclosed in U.S. Pat. No. 3,432,483. The process comprises the sequential steps of feeding to the polymer, water and a water-soluble block copolymer of ethylene oxide and propylene oxide surfactant into a dispersion zone; vigorously agitating the mixture under elevated temperature and pressure to form a dispersion of the molten polymer; withdrawing a portion of the dispersion and cooling to a temperature below the melting point of said polymer to form solid, finely divided polymer particles in the dispersion; reducing the pressure of said cooled dispersion to atmospheric pressure; separating the solid polymer particles from the surfactant solution phase and washing; drying the washed polymer particles; and recovering dry, finely divided powder.
While it is possible to produce a wide range of fine powders using such procedures, the method is not adaptable for use with all resins. As the melt index of a resin approaches 1, it becomes increasingly difficult to achieve the type of dispersion necessary to form fine powders. Dispersions having droplets of the size necessary for the production of fine powders cannot be formed with fractional melt flow rate resins, i.e., resins having a melt index less than 1. This is believed to be due, in part, to the high molecular weights of such resins. The relationship of melt flow rate to molecular weight and the inability to form dispersions suitable for the production of fine powders with low melt flow rate resins is discussed in U.S. Pat. No. 3,746,681.
It would be advantageous if fine powders of low melt flow rate resin powders could be produced utilizing a ispersion process, particularly if the particles had a relatively narrow particle size distribution and were spherical in shape. Coatings obtained using such powders would be expected to have improved thermal stability, improved creep resistance, improved chemical resistance and other desirable properties. A process is disclosed in our copending application Ser. No. 784,862, filed Oct. 30, 1991, for producing microfine fractional melt flow rate powders from olefin copolymers. For the process, an olefin copolymer resin having a melt flow rate greater than 1 is dispersed and the melt flow rate of the resin is then lowered during the powder-forming process.
The process of application Serial No. 784,862 pending involves combining an olefin copolymer having a melt index greater than 1 with 4 to 50 percent, based on the weight of the olefin copolymer, of a nonionic surfactant which is a block copolymer of ethylene oxide and propylene oxide, 0.001 to 10 percent, based on the weight of the olefin copolymer, of a catalyst selected from the group consisting of organic bases, mineral or carboxylic acids, organic titanates and complexes or carboxylates of lead, cobalt, iron, nickel and tin, and a polar liquid medium which is not a solvent for the olefin copolymer and which does not react with any of the foregoing ingredients under the conditions employed, the weight ratio of the polar liquid medium to the olefin copolymer ranging from 0.8:1 to 9:1; heating the mixture to a temperature above the melting point of the olefin copolymer; dispersing the mixture to form droplets of the desired size; maintaining the dispersion for a period of time sufficient to achieve the desired reduction of the melt index; cooling the dispersion to below the melting point of the olefin copolymer; and recovering the olefin copolymer powder. Olefin copolymers employed for the process of the invention are derived from C.sub.2-8 .alpha.-olefins and unsaturated alkoxysilanes, e.g., vinyltrialkoxysilanes.
Ethylene/vinylalkoxysilane copolymers are known. They are disclosed in U.S. Pat. Nos. 3,225,018 and 3,392,156. In U.S. Pat. No. 3,392,156 it is also disclosed that the ethylene/vinyltrialkoxysilane copolymers can be used in finely divided form where the copolymer has an average size of less than about 10 mesh and preferably in the range of about 150 to 2000 microns. While the reference states that the finely divided material may be prepared by mechanical grinding, solution or dispersion techniques or other methods, no details are provided. Furthermore, it is a requirement of the process that the products be mechanically worked to obtain a reduction of melt index and an increase in stress cracking resistance. Melt indexes obtained after mechanical working range from 7.95 to zero.
While the "one-step" procedure for producing microfine fractional melt flow rate powders disclosed in our above-identified copending application is highly useful, it would be advantageous to have a process whereby the melt flow rate of polymer powders could be reduced independent of the powder forming operation. This would enable processors to "customize" the melt flow rate of the powders to their specific application. It could also provide better control of the crosslinking. By having the crosslinking take place outside the powder-forming reactor, fouling or corrosion of the primary reactor caused by the presence of crosslinking catalysts could be avoided. It would be particularly useful if the melt flow reduction could be performed on the powders without substantially changing the particle size or particle size distribution.